With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary cells or batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Also, in consideration of global environments, electric cars and hybrid cars have been recently developed and put into practice, so that there is an increasing demand for lithium ion secondary cells for large size applications having excellent storage performance. Under these circumstances, lithium ion secondary cells having advantages such as large charge/discharge capacity and good storage performance have been noticed.
Hitherto, as cathode materials useful for high energy-type lithium ion secondary cells exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinet structure, LiMnO2 having a zigzag layer structure, LiCoO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary cells using these cathode materials, lithium ion secondary cells using LiNiO2 have been noticed because of large charge/discharge capacity thereof. However, the materials tend to be deteriorated in thermal stability and charge/discharge cycle durability upon charging, and, therefore, it has been required to further improve properties thereof.
Specifically, when lithium ions are de-intercalated from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li de-intercalated reaches 0.45, the crystal structure of such a lithium-de-intercalated region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further de-intercalated of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary cell tends to suffer from poor cycle characteristics or occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage performance of the cell. To solve these problems, there have been made studies on materials formed by adding Co and Al to a part of Ni of LiNiO2. However, these materials have still failed to solve the above-described problems. Therefore, it has still been required to provide a Li—Ni composite oxide having a more stabilized crystal structure.
In addition, since the particles of the Li—Ni composite oxide have a small primary particle diameter, in order to obtain a Li—Ni composite oxide having a high packing density, it is required to control properties of the Li—Ni composite oxide such that they are capable of forming densely aggregated secondary particles. However, the Li—Ni composite oxide in the form of secondary particles tends to suffer from breakage of the secondary particles owing to compression upon production of an electrode therefrom and is, therefore, increased in surface area, so that the resulting secondary cell tends to undergo promoted reaction between the composite oxide and an electrolyte solution upon storage in a charged state under a high temperature condition, resulting in formation of a non-conductive material film on a surface of the electrode and, therefore, increase in electric resistance of the secondary cell. Also, when impurities such as lithium sulfate are present in the Li—Ni composite oxide, there tend to arise the problems such as incomplete crystal growth of the Li—Ni composite oxide and formation of a non-conductive material film on a surface of the electrode owing to undesirable decomposition reaction of the impurities during a charge/discharge cycle thereof, resulting in increase in electric resistance of the secondary cell upon storage in a charged state under a high temperature condition. For these reasons, in order to ensure high storage performance of the secondary cell under a high temperature condition, it is required to not only obtain a Li—Ni composite oxide having a less content of impurities, but also suppress change in average particle diameter of the cathode material between before and after compressing the material upon production of the electrode therefrom while maintaining a high electrode density, and prevent the particles thereof from suffering from breakage.
Further, in the process for producing the Li—Ni composite oxide, in order to obtain the Li—Ni composite oxide having a high packing density and a stable crystal structure, it is required to use Ni composite hydroxide particles which are well controlled in properties, crystallinity and contents of impurities, and calcine the particles under the condition which is free from inclusion of Ni2+ into Li sites thereof.
More specifically, it is required to provide Li—Ni composite oxide capable of exhibiting a high packing density, a stable crystal structure and excellent storage performance as a cathode material for a non-aqueous electrolyte secondary cell.
Hitherto, in order to improve various properties such as stabilization of a crystal structure and charge/discharge cycle characteristics, various improvements of LiNiO2 particles have been attempted. For example, there are known the technique of stabilizing a crystal structure of LiNiO2 by adding other kinds of metals to Ni sites thereof (Patent Document 1); the technique of improving a tap density of Ni—Co hydroxide used for production of the Li—Ni composite oxide to reduce a content of residual impurities therein (Patent Document 2); the technique of controlling a cumulative volume-based particle size distribution of the Li—Ni composite oxide to a limited range to obtain a cathode material having a large volume (capacity) density, a high safety, an excellent coating uniformity, an excellent charge/discharge cycle durability and low-temperature performance (Patent Document 3); the technique of not only increasing a rate of occupation of Li sites in the Li—Ni composite oxide but also reducing an amount of change in BET specific surface area upon subjecting the Li—Ni composite oxide to wasting treatment to enhance an initial capacity thereof (Patent Document 4); etc.    Patent Document 1: Japanese Patent Application Laid-open (KOKAI) No. 5-242891 (1993)    Patent Document 2: Japanese Patent Application Laid-open (KOKAI) No. 2001-106534    Patent Document 3: PCT Pamphlet WO 01/092158    Patent Document 4: Japanese Patent Application Laid-open (KOKAI) No. 2004-171961